1Say you've got a big slow raid 6, and an X-25E or three. Wouldn't it be
2nice if you could use them as cache... Hence bcache.
34Wiki and git repositories are at:
5http://bcache.evilpiepirate.org6http://evilpiepirate.org/git/linux-bcache.git7http://evilpiepirate.org/git/bcache-tools.git89It's designed around the performance characteristics of SSDs - it only allocates
10in erase block sized buckets, and it uses a hybrid btree/log to track cached
11extants (which can be anywhere from a single sector to the bucket size). It's
12designed to avoid random writes at all costs; it fills up an erase block
13sequentially, then issues a discard before reusing it.
1415Both writethrough and writeback caching are supported. Writeback defaults to
16off, but can be switched on and off arbitrarily at runtime. Bcache goes to
17great lengths to protect your data - it reliably handles unclean shutdown. (It
18doesn't even have a notion of a clean shutdown; bcache simply doesn't return
19writes as completed until they're on stable storage).
2021Writeback caching can use most of the cache for buffering writes - writing
22dirty data to the backing device is always done sequentially, scanning from the
23start to the end of the index.
2425Since random IO is what SSDs excel at, there generally won't be much benefit
26to caching large sequential IO. Bcache detects sequential IO and skips it;
27it also keeps a rolling average of the IO sizes per task, and as long as the
28average is above the cutoff it will skip all IO from that task - instead of
29caching the first 512k after every seek. Backups and large file copies should
30thus entirely bypass the cache.
3132In the event of a data IO error on the flash it will try to recover by reading
33from disk or invalidating cache entries. For unrecoverable errors (meta data
34or dirty data), caching is automatically disabled; if dirty data was present
35in the cache it first disables writeback caching and waits for all dirty data
36to be flushed.
3738Getting started:
39You'll need make-bcache from the bcache-tools repository. Both the cache device
40and backing device must be formatted before use.
41 make-bcache -B /dev/sdb
42 make-bcache -C /dev/sdc
4344make-bcache has the ability to format multiple devices at the same time - if
45you format your backing devices and cache device at the same time, you won't
46have to manually attach:
47 make-bcache -B /dev/sda /dev/sdb -C /dev/sdc
4849bcache-tools now ships udev rules, and bcache devices are known to the kernel
50immediately. Without udev, you can manually register devices like this:
5152 echo /dev/sdb > /sys/fs/bcache/register
53 echo /dev/sdc > /sys/fs/bcache/register
5455Registering the backing device makes the bcache device show up in /dev; you can
56now format it and use it as normal. But the first time using a new bcache
57device, it'll be running in passthrough mode until you attach it to a cache.
58See the section on attaching.
5960The devices show up as:
6162 /dev/bcache<N>
6364As well as (with udev):
6566 /dev/bcache/by-uuid/<uuid>
67 /dev/bcache/by-label/<label>
6869To get started:
7071 mkfs.ext4 /dev/bcache0
72 mount /dev/bcache0 /mnt
7374You can control bcache devices through sysfs at /sys/block/bcache<N>/bcache .
7576Cache devices are managed as sets; multiple caches per set isn't supported yet
77but will allow for mirroring of metadata and dirty data in the future. Your new
78cache set shows up as /sys/fs/bcache/<UUID>
7980ATTACHING:
8182After your cache device and backing device are registered, the backing device
83must be attached to your cache set to enable caching. Attaching a backing
84device to a cache set is done thusly, with the UUID of the cache set in
85/sys/fs/bcache:
8687 echo <CSET-UUID> > /sys/block/bcache0/bcache/attach
8889This only has to be done once. The next time you reboot, just reregister all
90your bcache devices. If a backing device has data in a cache somewhere, the
91/dev/bcache<N> device won't be created until the cache shows up - particularly
92important if you have writeback caching turned on.
9394If you're booting up and your cache device is gone and never coming back, you
95can force run the backing device:
9697 echo 1 > /sys/block/sdb/bcache/running
9899(You need to use /sys/block/sdb (or whatever your backing device is called), not
100/sys/block/bcache0, because bcache0 doesn't exist yet. If you're using a
101partition, the bcache directory would be at /sys/block/sdb/sdb2/bcache)
102103The backing device will still use that cache set if it shows up in the future,
104but all the cached data will be invalidated. If there was dirty data in the
105cache, don't expect the filesystem to be recoverable - you will have massive
106filesystem corruption, though ext4's fsck does work miracles.
107108ERROR HANDLING:
109110Bcache tries to transparently handle IO errors to/from the cache device without
111affecting normal operation; if it sees too many errors (the threshold is
112configurable, and defaults to 0) it shuts down the cache device and switches all
113the backing devices to passthrough mode.
114115 - For reads from the cache, if they error we just retry the read from the
116 backing device.
117118 - For writethrough writes, if the write to the cache errors we just switch to
119 invalidating the data at that lba in the cache (i.e. the same thing we do for
120 a write that bypasses the cache)
121122 - For writeback writes, we currently pass that error back up to the
123 filesystem/userspace. This could be improved - we could retry it as a write
124 that skips the cache so we don't have to error the write.
125126 - When we detach, we first try to flush any dirty data (if we were running in
127 writeback mode). It currently doesn't do anything intelligent if it fails to
128 read some of the dirty data, though.
129130TROUBLESHOOTING PERFORMANCE:
131132Bcache has a bunch of config options and tunables. The defaults are intended to
133be reasonable for typical desktop and server workloads, but they're not what you
134want for getting the best possible numbers when benchmarking.
135136 - Bad write performance
137138 If write performance is not what you expected, you probably wanted to be
139 running in writeback mode, which isn't the default (not due to a lack of
140 maturity, but simply because in writeback mode you'll lose data if something
141 happens to your SSD)
142143 # echo writeback > /sys/block/bcache0/cache_mode
144145 - Bad performance, or traffic not going to the SSD that you'd expect
146147 By default, bcache doesn't cache everything. It tries to skip sequential IO -
148 because you really want to be caching the random IO, and if you copy a 10
149 gigabyte file you probably don't want that pushing 10 gigabytes of randomly
150 accessed data out of your cache.
151152 But if you want to benchmark reads from cache, and you start out with fio
153 writing an 8 gigabyte test file - so you want to disable that.
154155 # echo 0 > /sys/block/bcache0/bcache/sequential_cutoff
156157 To set it back to the default (4 mb), do
158159 # echo 4M > /sys/block/bcache0/bcache/sequential_cutoff
160161 - Traffic's still going to the spindle/still getting cache misses
162163 In the real world, SSDs don't always keep up with disks - particularly with
164 slower SSDs, many disks being cached by one SSD, or mostly sequential IO. So
165 you want to avoid being bottlenecked by the SSD and having it slow everything
166 down.
167168 To avoid that bcache tracks latency to the cache device, and gradually
169 throttles traffic if the latency exceeds a threshold (it does this by
170 cranking down the sequential bypass).
171172 You can disable this if you need to by setting the thresholds to 0:
173174 # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us
175 # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us
176177 The default is 2000 us (2 milliseconds) for reads, and 20000 for writes.
178179 - Still getting cache misses, of the same data
180181 One last issue that sometimes trips people up is actually an old bug, due to
182 the way cache coherency is handled for cache misses. If a btree node is full,
183 a cache miss won't be able to insert a key for the new data and the data
184 won't be written to the cache.
185186 In practice this isn't an issue because as soon as a write comes along it'll
187 cause the btree node to be split, and you need almost no write traffic for
188 this to not show up enough to be noticeable (especially since bcache's btree
189 nodes are huge and index large regions of the device). But when you're
190 benchmarking, if you're trying to warm the cache by reading a bunch of data
191 and there's no other traffic - that can be a problem.
192193 Solution: warm the cache by doing writes, or use the testing branch (there's
194 a fix for the issue there).
195196SYSFS - BACKING DEVICE:
197198Available at /sys/block/<bdev>/bcache, /sys/block/bcache*/bcache and
199(if attached) /sys/fs/bcache/<cset-uuid>/bdev*
200201attach
202 Echo the UUID of a cache set to this file to enable caching.
203204cache_mode
205 Can be one of either writethrough, writeback, writearound or none.
206207clear_stats
208 Writing to this file resets the running total stats (not the day/hour/5 minute
209 decaying versions).
210211detach
212 Write to this file to detach from a cache set. If there is dirty data in the
213 cache, it will be flushed first.
214215dirty_data
216 Amount of dirty data for this backing device in the cache. Continuously
217 updated unlike the cache set's version, but may be slightly off.
218219label
220 Name of underlying device.
221222readahead
223 Size of readahead that should be performed. Defaults to 0. If set to e.g.
224 1M, it will round cache miss reads up to that size, but without overlapping
225 existing cache entries.
226227running
228 1 if bcache is running (i.e. whether the /dev/bcache device exists, whether
229 it's in passthrough mode or caching).
230231sequential_cutoff
232 A sequential IO will bypass the cache once it passes this threshold; the
233 most recent 128 IOs are tracked so sequential IO can be detected even when
234 it isn't all done at once.
235236sequential_merge
237 If non zero, bcache keeps a list of the last 128 requests submitted to compare
238 against all new requests to determine which new requests are sequential
239 continuations of previous requests for the purpose of determining sequential
240 cutoff. This is necessary if the sequential cutoff value is greater than the
241 maximum acceptable sequential size for any single request.
242243state
244 The backing device can be in one of four different states:
245246 no cache: Has never been attached to a cache set.
247248 clean: Part of a cache set, and there is no cached dirty data.
249250 dirty: Part of a cache set, and there is cached dirty data.
251252 inconsistent: The backing device was forcibly run by the user when there was
253 dirty data cached but the cache set was unavailable; whatever data was on the
254 backing device has likely been corrupted.
255256stop
257 Write to this file to shut down the bcache device and close the backing
258 device.
259260writeback_delay
261 When dirty data is written to the cache and it previously did not contain
262 any, waits some number of seconds before initiating writeback. Defaults to
263 30.
264265writeback_percent
266 If nonzero, bcache tries to keep around this percentage of the cache dirty by
267 throttling background writeback and using a PD controller to smoothly adjust
268 the rate.
269270writeback_rate
271 Rate in sectors per second - if writeback_percent is nonzero, background
272 writeback is throttled to this rate. Continuously adjusted by bcache but may
273 also be set by the user.
274275writeback_running
276 If off, writeback of dirty data will not take place at all. Dirty data will
277 still be added to the cache until it is mostly full; only meant for
278 benchmarking. Defaults to on.
279280SYSFS - BACKING DEVICE STATS:
281282There are directories with these numbers for a running total, as well as
283versions that decay over the past day, hour and 5 minutes; they're also
284aggregated in the cache set directory as well.
285286bypassed
287 Amount of IO (both reads and writes) that has bypassed the cache
288289cache_hits
290cache_misses
291cache_hit_ratio
292 Hits and misses are counted per individual IO as bcache sees them; a
293 partial hit is counted as a miss.
294295cache_bypass_hits
296cache_bypass_misses
297 Hits and misses for IO that is intended to skip the cache are still counted,
298 but broken out here.
299300cache_miss_collisions
301 Counts instances where data was going to be inserted into the cache from a
302 cache miss, but raced with a write and data was already present (usually 0
303 since the synchronization for cache misses was rewritten)
304305cache_readaheads
306 Count of times readahead occurred.
307308SYSFS - CACHE SET:
309310Available at /sys/fs/bcache/<cset-uuid>
311312average_key_size
313 Average data per key in the btree.
314315bdev<0..n>
316 Symlink to each of the attached backing devices.
317318block_size
319 Block size of the cache devices.
320321btree_cache_size
322 Amount of memory currently used by the btree cache
323324bucket_size
325 Size of buckets
326327cache<0..n>
328 Symlink to each of the cache devices comprising this cache set.
329330cache_available_percent
331 Percentage of cache device which doesn't contain dirty data, and could
332 potentially be used for writeback. This doesn't mean this space isn't used
333 for clean cached data; the unused statistic (in priority_stats) is typically
334 much lower.
335336clear_stats
337 Clears the statistics associated with this cache
338339dirty_data
340 Amount of dirty data is in the cache (updated when garbage collection runs).
341342flash_vol_create
343 Echoing a size to this file (in human readable units, k/M/G) creates a thinly
344 provisioned volume backed by the cache set.
345346io_error_halflife
347io_error_limit
348 These determines how many errors we accept before disabling the cache.
349 Each error is decayed by the half life (in # ios). If the decaying count
350 reaches io_error_limit dirty data is written out and the cache is disabled.
351352journal_delay_ms
353 Journal writes will delay for up to this many milliseconds, unless a cache
354 flush happens sooner. Defaults to 100.
355356root_usage_percent
357 Percentage of the root btree node in use. If this gets too high the node
358 will split, increasing the tree depth.
359360stop
361 Write to this file to shut down the cache set - waits until all attached
362 backing devices have been shut down.
363364tree_depth
365 Depth of the btree (A single node btree has depth 0).
366367unregister
368 Detaches all backing devices and closes the cache devices; if dirty data is
369 present it will disable writeback caching and wait for it to be flushed.
370371SYSFS - CACHE SET INTERNAL:
372373This directory also exposes timings for a number of internal operations, with
374separate files for average duration, average frequency, last occurrence and max
375duration: garbage collection, btree read, btree node sorts and btree splits.
376377active_journal_entries
378 Number of journal entries that are newer than the index.
379380btree_nodes
381 Total nodes in the btree.
382383btree_used_percent
384 Average fraction of btree in use.
385386bset_tree_stats
387 Statistics about the auxiliary search trees
388389btree_cache_max_chain
390 Longest chain in the btree node cache's hash table
391392cache_read_races
393 Counts instances where while data was being read from the cache, the bucket
394 was reused and invalidated - i.e. where the pointer was stale after the read
395 completed. When this occurs the data is reread from the backing device.
396397trigger_gc
398 Writing to this file forces garbage collection to run.
399400SYSFS - CACHE DEVICE:
401402Available at /sys/block/<cdev>/bcache
403404block_size
405 Minimum granularity of writes - should match hardware sector size.
406407btree_written
408 Sum of all btree writes, in (kilo/mega/giga) bytes
409410bucket_size
411 Size of buckets
412413cache_replacement_policy
414 One of either lru, fifo or random.
415416discard
417 Boolean; if on a discard/TRIM will be issued to each bucket before it is
418 reused. Defaults to off, since SATA TRIM is an unqueued command (and thus
419 slow).
420421freelist_percent
422 Size of the freelist as a percentage of nbuckets. Can be written to to
423 increase the number of buckets kept on the freelist, which lets you
424 artificially reduce the size of the cache at runtime. Mostly for testing
425 purposes (i.e. testing how different size caches affect your hit rate), but
426 since buckets are discarded when they move on to the freelist will also make
427 the SSD's garbage collection easier by effectively giving it more reserved
428 space.
429430io_errors
431 Number of errors that have occurred, decayed by io_error_halflife.
432433metadata_written
434 Sum of all non data writes (btree writes and all other metadata).
435436nbuckets
437 Total buckets in this cache
438439priority_stats
440 Statistics about how recently data in the cache has been accessed.
441 This can reveal your working set size. Unused is the percentage of
442 the cache that doesn't contain any data. Metadata is bcache's
443 metadata overhead. Average is the average priority of cache buckets.
444 Next is a list of quantiles with the priority threshold of each.
445446written
447 Sum of all data that has been written to the cache; comparison with
448 btree_written gives the amount of write inflation in bcache.